While the background of the present invention is discussed in terms of plasma display panel (PDP) applications, it is understood that the present invention is also useful in flat panel display applications, in general, as well as in black ceramic dielectrics.
The PDP typically comprises a pair of forward and backward insulation substrates arranged in opposition to each other to form a plurality of cells as display elements each defined by the insulation substrates supported with a constant interval and cell barriers arranged between the insulation substrates, two crossing electrodes disposed on internal surfaces of the insulation substrates with a dielectric layer interposed between the electrodes which cause electric discharge in a plurality of cells by application of an alternating current. Due to this application of alternating current, phosphor screens formed on the wall surface of the cell barrier emit light and display images which are passed through the transparent insulation substrate (typically called the front glass substrate or plate).
One area of concern for PDP manufacturers is display contrast, which affects the ultimate picture viewed by the consumer. To improve the display contrast, it is essential to decrease the reflection of external light from the electrodes and conductors arranged on the front glass substrate of the PDP device. This reflection decrease can be accomplished by making the electrodes and conductors black as viewed through the front plate of the display. The black pigments are used to improve cosmetics of applied circuitry by masking the circuit behind a layer of black enamel. In display applications, compositions containing black pigments are deposited in a way that enhances the contrast between the lighted pixels and the unlit areas of the display when the panel is being actively viewed.
Another area of concern for PDP manufacturers is of an environmental nature and is the lead and cadmium contained in some conventional black conductor compositions and black electrodes used in PDP devices. It is desirable to reduce and/or eliminate the lead and cadmium contained in the black conductor compositions and electrodes while still maintaining the required physical and electrical properties of the compositions and electrodes.
U.S. Pat. Nos. 5,851,732 and 6,075,319 to Kanda et al. disclose a photoformable black electrode comprising a conductive layer of at least one of RuO2, ruthenium based polynary oxide or mixtures thereof formed between the substrate and conductor electrode arrangement.
Bismuth ruthenium pyrochlore and lead bismuth ruthenium pyrochlore, and other chemical compounds, have been used as black pigments.
U.S. Patent Publication No. 2006-0231806 A1, discloses the use of bismuth glasses and bismuth ruthenium pyrochlore, copper bismuth ruthenium pyrochlore, and gadolinium bismuth pyrochhlore as pigments, preferably with surface areas less than 20 m2/g.
Additionally, some prior art compositions have utilized “spinels” as pigment. Spinels, as used herein, are mineral oxides defined by the formula AB2O4, where A and B represent cations. While the ideal spinel formula is MgAl2O4, some 30 elements, with valences from 1 to 6, are known to substitute in the A or B cation sites, resulting in well over 150 synthetic compounds having the spinel crystal structure. Spinels have a pointed octahedral, crystal habit, and also form a dendritic snowflake form (i.e., a mineral crystallizing in another mineral in the form of a branching or treelike mark) in rapidly chilled high-temperature slags and lavas. The named spinel minerals that have so far been recorded in nature are oxides that occur as a matrix of A2+ versus B3+ cations.
Rangavittal N. et al., Eur. J. Solid State Inorg. Chem., v. 31, p. 409 (1994) have prepared and reported the cation distributions of several derivatives of the cubic oxide γ-Bi2O3, where the Bi atoms are replaced by any one of the following metals Co, Mn, Fe, Ti, Ni, or Pb. In particular, they showed that it was possible to prepare a wide range of compounds where a substantial portion of the bismuth could be replaced with cobalt.
The present inventors have developed novel pigment compositions that improve display contrast and avoid the above described environmental problems.